Philosophy of law is a branch of philosophy and jurisprudence which studies basic questions about law and legal systems, such as "what is the law?", "what are the criteria for legal validity?", "what is the relationship between law and morality?", and many other similar questions.

Natural law theory asserts that there is an essential connection between law and morality. This view is frequently summarized by the maxim: an unjust law is not a true law.

Legal positivism is the view that the law is defined by the social rules or practices that identify certain norms as laws. Historically, the most important legal positivist theory was developed by Jeremy Bentham, whose views were popularized by his student, John Austin. Austin's version of legal positivism was based on the notion that the law is the command of the sovereign backed by the threat of punishment.

Legal realism is the view that that the law should be understood as it is practiced in the courts, law offices, and police stations, rather than as it is set forth in statutes or learned treatises.

In the twentieth century, two great legal positivists had a profound influence on the philosophy of law. On the continent, Hans Kelsen was the most influential theory, and his notion of a grundnorm or ultimate and basic legal norm is still influential. In the Anglophone world, the most influential figure was H.L.A. Hart, who argued that the law should be understood as a system of social rules. Hart's theory, although widely admired, was criticized by a variety of late twentieth century philosophers of law, including Ronald Dworkin, John Finnis, and Joseph Raz.

In recent years, debates over the nature of law have focused on two issues. The first of these is a debate within legal positivism between two schools of thought. The first school is sometimes called exclusive legal positivism, and it is associated with the view that the legal validity of a norm can never depend on its moral correctness. The second school is labeled inclusive legal positivism, and it is associated with the view that moral considerations may determine the legal validity of a norm, but that it not necessary that this is the case. Any theory that held that there was a necessary connection between law and morality would not be a form of legal positivism.

The second important debate in recent years concerns interpretivism--a view that is strongly associated with Ronald Dworkin. An interpretivist theory of law holds that legal rights and duties are determined by the best interpretation of the political practices of a particular community. Interpretation, according to Dworkin, has two dimensions. To count as an interpretation, the reading of a text must meet the criterion of fit. But of those interpretations that fit, Dworkin maintains that the correct interpretation is the one that puts the political practices of the community in their best light, or makes of them the best that they can be.

Utilitarianism is the view that the laws should be crafted so as to produce the best consequences. Historically, utilitarian thinking about law is associated with the great philosopher, Jeremy Bentham. In contemporary legal theory, the utilitarian approach is frequently championed by scholars who work in the law and economics tradition.

Deontology is the view that the laws should protect individual autonomy, liberty, or rights. The philosopher Immanuel Kant formulated a deontological theory of law. A contemporary deontological approach can be found in the work of the liberal philosopher Ronald Dworkin.

Aretaic moral theories such as contemporary virtue ethics emphasize the role of character in morality. Virtue jurisprudence is the view that the laws should promote the development of virtuous characters by citizens. Historically, this approach is associated with Aristotle. Contemporary virtue jurisprudence is inspired by philosophical work on virtue ethics.